Using only its two forelimbs to propel itself on land, Ichthyostega may represent how first limbed animals transitioned from swimming like a fish to walking.

All living four limbed animals, called tetrapods, owe a debt of gratitude to our fishlike ancestors that first braved terra firma more than 350 million years ago. A new study reveals that Ichthyostega, among the best known of those amphibious pioneers, did not walk around on four sturdy legs as do salamanders today. Instead, the dog size predator used its two forelimbs to propel itself along the ground as if on crutches, with its paddle like hind legs barely touching the ground. The style of locomotion recalls that of modern mudskipper fish wriggling about on tidal flats.

The insights into Ichthyostega’s gait come from three dimensional computer reconstructions based on dozens of fossil specimens. Using microcomputerized tomography scans, paleobiologist Stephanie E. Pierce of the Royal Veterinary College and the University of Cambridge, England, and colleagues painstakingly extricated the forms of embedded bone pieces from their rock matrix and reassem bled a skeleton in virtual space. Then, they analyzed the model skeleton by gauging the range of motion of the limb joints. To interpret the results in a locomotion context, the researchers compared Ichthyostega with today’s crocodiles, otters, platypuses, salamanders, and seals.

Relative to living forms, Ichthyostega had restricted rotation in the shoulder and hip joints. In addition, the hip joints were offset by forty-five degrees from the horizontal, preventing the ancient vertebrate from rotating its hind limb into a position to plant its rear feet on the ground and lift its pelvis. The findings suggest the ancient beast could not walk on land, but rather hauled itself about using synchronous rowing motions of its forelimbs, most likely using its hind limbs for swimming.

While slow on land, in the water the big headed, fanged toothed Ichthyostega “was a force to be reckoned with,” says Pierce. The question remains whether Ichthyostega’s method of foraying onto land served as the pivotal first step, so to speak, that allowed for the rise of terrestrial vertebrates. “We need to examine other early tetrapods to see how representative Ichthyostega is,” says Pierce. “Then we can start to build a picture of how the first limbed animals transitioned from swimming like a fish to walking on land.” (Nature)